An image transfer and contact fixing device is described in European Patent Application No. 0149860 as having a photoconductive element and a pressure roller for pressing the receiving material and fixing the image thereon. Both of these are pressed against the transfer roller at different points. In this device, the nip between the photoconductive element and the transfer roller is situated straight opposite from the nip between the transfer roller and the pressure roller.
In a device with this configuration, the force with which the photoconductive element is pressed against the transfer roller generally differs from the force with which the pressure roller is pressed against the transfer roller so that a resultant force is exerted on the transfer roller. This resultant force causes the transfer roller to sag if it is conventionally secured at its ends, with the net result that the pressure in the transfer nip is not sufficiently uniform and causes an irregular transfer. The irregular transfer can be further aggravated by the presence of a cleaning roller in the image transfer and fixing device such as is shown in European Patent Application No. 0149860. This roller causes extra sagging in a direction other than that already referred to above.
Various other configurations of image transfer and contact fixing devices are shown in U.S. Pat. No. 4,645,327 and Abstracts of Japanese Patent Application Nos. 59-53873, 59-168482, and 59-81667. These devices show the transfer roller forming various different angles with the photoconductive element, the pressure roller or a cleaning roller. In none of these devices, however, is there any discussion of forces let alone trying to minimize the resultant force on the transfer roller.
If receiving material is passed through a transfer nip in which the pressure is irregular due to sagging, the receiving material can easily be creased. The sagging phenomena causes the most problems in transfer devices equipped with a long transfer roller for copying onto receiving material having a large format.
It would be desirable, therefore, to have an image transfer and contact fixing device which eliminates the sagging problem.